The present invention relates to a barrier film for a flexible copper substrate and a sputtering target for forming such barrier film capable of effectively inhibiting the diffusion of copper to a resin film such as polyimide.
Conventionally, a copper layer is formed on a resin film such as polyimide, which is to become the base film, upon manufacturing a flexible copper substrate. Specifically, a copper seed layer is formed on the polyimide film by sputtering or electroless deposition, and a thick copper plated layer is formed thereon. Thereafter, the copper is subject to etching so as to form a copper circuit pattern.
The problem here is that the copper formed on the polyimide film easily diffuses (migrates) in the polyimide film, and causes a short circuit in the wiring on the circuit board.
In order to inhibit this kind of diffusion of Cu to the polyimide film, proposed is a method of forming in advance a barrier layer for preventing the diffusion of Cu on the polyimide film, and forming a Cu seed layer and a thick Cu plated layer thereon.
As a representative example, there is technology which forms a Ni—Cr alloy barrier layer. See Japanese Patent Laid-Open Publication No. 2002-252257.
Nevertheless, when the temperature rises to about 200 to 300° C., diffusion of Cu to the polyimide film can still be acknowledged. In addition, when the wiring pitch becomes narrower than 30 μm, it is not possible to prevent diffusion to the polyimide layer with a conventional barrier layer, and it has become known that this is not necessarily effective.
As a means for preventing the foregoing problem, it is conceivable that the barrier characteristics can be improved by thickening the conventional barrier layer. Nevertheless, when the thickness exceeds a certain value, another problem occurs in that the barrier film would peel from the polyimide film. Therefore, this method was insufficient to become a fundamental solution.
As another proposal, suggested is a method of forming a thermoplastic polyimide layer on a thermosetting polyimide base film, covering this with a barrier metal formed from at least one type of metal selected from Ni, Cr, Co and Mo, heating and fluidizing the thermoplastic resin, and increasing the bonding strength between the thermoplastic polyimide and the barrier metal. See Japanese Patent Laid-Open Publication No. 2002-280684.
Nevertheless, in this case, since the foregoing method does not fundamentally solve the diffusion of the barrier metal, this problem still needs to be solved.